The Louis XIV's codebooks, The Great Cipher () ()

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The Louis XIV's codebooks

Introduction

The nomenclator, was the first method of encryption used from the Renaissance to allow dialogue between the nascent embassies. Nomenclator evolved into codebook. One of the main technological innovations in codebooks was the invention of double tables: one table to encrypt and another to decrypt. This invention was made by the Rossignol cryptanalysts attached to the service of King Louis XIV towards the end of the 17th century. This double tables coding technique is also called the Great Cipher. In this web page we will describe its invention, its characteristics, the previous methods and give examples of codebooks from the time of Louis XIV using one or two tables.

The codebooks, The Great Cipher

The codebooks

From the beginning of the use of cryptography during the renaissance, the method used was the nomenclators. They were, at the beginning, only simple substitution alphabets. The Cryptanalysts of the time easily cracked cryptograms by discovering vowels. In fact, to counter this attack, the same vowel was coded in different ways (e = 31, 55, 88). Later, this technique was extended to all letters. Always in the spirit of countering frequency analyses, we coded not only the letters, but also the syllables (ba, be bi, bo, bu, ...), the most frequent words as well as certain proper nouns (king, pope, emperor, etc.). As a result, the number of codegroups increased from a few dozen groups to several hundred.

Representation of codegroups

At the very beginning, a codegroup was a single letter, but as soon as each vowel and then each letter was represented by several codegroups, the letters no longer sufficed. Geometric symbols were then used (square, triangle, etc.), the symbols from alchemy and many other characters (including digits and numbers) and even characters were invented (see the codebook for 1643) .

As long as the number of codegroups was low, this technique was sufficient. But with the increase in the number of groups, the use of special characters became a real hassle for the cipher office. We then turned to the use of numbers (110. 679. 443. 718.). Sometimes the same number could have several meanings by surmounting them with symbols (//, -, .., ~, ^, \) as in the codebook of 1676. But in the end, the exclusive use of numbers became the rule, hence the terms cipher (for encrypt or encode), decipher (for decoding or decrypt), cipher clerk (the person who encode and decode) , ...

The two-part codebooks (The Great Cipher)

A one part codebook (a nomenclator) was written on a single sheet. To aid in decoding, codegroups were written in numerical order. To aid in encoding, the meaning of these groups also followed an order; for example, 43 = ra, 44 = re, 45 = ri, 46 = ro, 47 = ru. So it was easy to encode and decode.

Unfortunately this order helps the cryptanalyst: if he found the meaning of some codegrup. For example, codegroups 43 (ra) and 44 (re) he can deduce the meaning of the following codegroups (45 = ri, 46 = ro, 47 = ru).

It is to break this regularity that codebooks with double tables (two part codebooks) were invented: a table to encrypt and another to decrypt. The table for decrypting contains the list of codegroups in numerical order: … 43 = ra, 44 = for, 45 = x, 46 = pu … The table for encrypting contains the groups in alphabetical order: pi = 314, po = 110, pour = 44, pu = 46, … With this method, even if the cryptanalyst has deciphered some codegroups, he has no clues to decipher others.

Note: Personally I confuse the term Great Cipher" with two part codebook. I.E. the use of Encryption and Decryption tables. The term "Great Cipher" in fact, was coined by Bazeries in the late 19th century when he deciphered cipher letters from Louis XIV exchanged with Marshal Catinat. He also coined the expression "Small Cipher" to describe a method that also uses two tables but has a lower number of codegroups and therefore provides less security and is therefore used for less strategic exchanges.

References